well, if you're putting two /casts in the one macro only one will ever work.
The only exception to this are spells/trinkets/items that dont trigger the Global CD.

Casting every x amount of seconds, unless you use a castsequence, would be classed as automation, thus has been removed from the game.
The best you could hope for would be
/castsequence [harm] reset=20 Chain Lightning, Lightning Bolt, Lightning Bolt, Lightning Bolt, Lightning Bolt, Lightning Bolt, Lightning Bolt, Lightning Bolt, Lightning Bolt, Lightning Bolt, Lightning Bolt
or something along those lines.

No real need to exists,nodead if its a damage spell and you're not casting through a focus or target, it's redundant.